There are heaps of reasons to wait until marriage. http://waitingtillmarriage.org/the-top-10-awesome-benefits-of-waiting-until-marriage/
Also, adolescents who wait are proven to have higher intelligence and waiting can really stop you from screwing up relationships and getting too distracted by...
It all depends on the people involved. My girlfriend and I live about 5 hours apart and so, seeing each other all the time and being preoccupied with her is less of an issue. I find that having a girlfriend is good for study because if you are both committed students and limit your time talking...